Chapter 9

State of the Art Treatment of Produced Water Rangarajan T. Duraisamy, Ali Heydari Beni and Amr Henni Additional information is available at the end of the chapter http://dx.doi.org/10.5772/53478

1. Introduction Produced water is water trapped during subsurface formations which is brought to the surface along with oil or gas. It contributes the largest volume of waste stream associated with oil and gas production. Globally, 77 billion bbl of water are produced per annum. The conventional methods to handle waste stream are reinjection into the well, direct discharge or reuse in case of thermal loop. Out of these, the most efficient way of handling produced water is to re-inject it into disposal wells. The disposal cost, which includes transportation cost, capital cost and infrastructure maintenance cost, may be as much as $4.00/bbl. On the other hand, many oil producing regions (West Texas, Middle East and the Central Asian Republics) have scarcity of potable water. An affordable water treatment process could convert produced water into an asset. The harmful effects of produced water and the depletion of usable water resources act as a driving force for the treatment of produced water. Produced water contains soluble and insoluble organic compounds, dissolved solids, production chemicals (corrosion inhibitors, surfactants etc.) and solid particles due to leaching of rocks and corrosion of pipelines. The methods available for treating produced water are physical, chemical, biological and membrane treatment processes. Stringent water quality parameters can be achieved efficiently through membrane processes. The most important advantages of using membrane processes are the ease of operation and little or no requirement of chemicals. Based on pore size, the membrane processes could be classified into Microfiltration (MF), Ultrafiltration (UF) and Nanofiltration (NF). The membranes are also classified as organic, inorganic and composite membranes. The primary disadvantage of using membranes is fouling. Irreversible and reversible foulings occur while treating produced water. The usage of appropriate pre-treatment process reduces the membrane fouling to a greater extent. Commercial treatment methods based on reverse osmosis and ion exchange processes are also discussed.

© 2013 Henni et al., licensee InTech. This is an open access chapter distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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2. Characteristics of produced water The physical and chemical properties of produced water depend on the geographic location of the field, the geological formation with which the produced water has been in contact for thousands of years, and the type of hydrocarbon product being produced. The main constituents of produced water are as follows:     

dissolved and dispersed oil compounds dissolved formation minerals production chemical compounds production solids (formation, corrosion, scale, bacteria, waxes, and asphaltenes) dissolved gases

Produced waters discharged from gas/condensate platforms are about 10 times more toxic than the produced waters discharged from oil wells, but, the volumes from gas production are much lower; hence the total impact may be less.

2.1. Constituents in produced water 1.

2.

3.

4.

5. 6.

Dispersed oil: Oil is an important contaminant in produced water since it can create potentially toxic effects near the discharge point. It can significantly contribute to Biological Oxygen Demand (BOD) and hence affects the aquatic or marine ecosystem. Usually the size of dispersed oil droplets would be 4-6 microns, but it may vary from 230 microns. The current treatment systems could recover oil droplets of size up-to 10 microns. Dissolved Organic Compounds: They include organic acids, polycyclic aromatic hydrocarbons (PAHs), phenols and volatiles. Volatile hydrocarbons can occur naturally in produced water. Concentrations of these compounds are usually higher in produced water from gas-condensate-producing platforms than in produced water from oilproducing platforms. Treatment Chemicals: They include biocides, reverse emulsion breakers, and corrosion inhibitors. Corrosion inhibitors can form stable emulsions. Some chemicals are highly toxic even at low concentrations such as 0.1 ppm. Produced Solids: They consist of precipitated solids (scales), sand and silt, carbonates, clays, corrosion products and other suspended solids produced from the formation and from well bore operations. Bacteria: Anaerobic bacteria present in produced water may lead to corrosion. Metals: Zinc, Lead, Manganese, Iron and Barium are the metals usually present in produced water. They are in general less toxic when compared to organic constituents. But they may precipitate to form undesired solids which hinder the treatment processes.

An example of key parameters of produced water is listed below in Table 1.

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3. Produced water management[1] a.

b. c. d.

Injection into oil wells: The produced water is injected into the same oil well from where it is obtained or transported to the discharge well at another location. The cost varies from $0.70 to $4.00. Direct discharge: The produced water is discharged directly as per the regulation norms. The cost varies from $0.03 to $0.05. Reuse in oil and gas operation: The produced water could be treated and used in the oil and gas processing industries. The cost varies from $0.04 to $0.07. Consumed in beneficial use: Treating produced water to convert it into an asset. The cost varies from $0.25 to $2.00.

Parameter Oil/grease(ppm) pH TSS(ppm) TDS(ppm) TOC(ppm) COD(ppm) Density(kg/m3) Arsenic(ppm) Lead(ppm) Chromium(ppm) Mercury(ppm) Oil droplet size(µm)

Natural Gas Produced Water 40 4.4-7.0 5500 360,000 67-38,000 120,000 1020 0.005-151 0.2-10.2 0.03 -2 to 30

Oil field Produced Water 560 4.3–10 1000 6554 1500 1220 1140 0.005–0.3 0.008-8.8 0.02–1.1 0.001–0.002

Table 1. Key parameters of importance in produced water treatments [1]

4. Treatment methods 4.1. Physical treatment 4.1.1. Physical adsorption Activated carbon, organoclay, copolymers, zeolite, resins are widely used to treat produced water. The combination of activated carbon and organoclays proved to be more efficient in removing total petroleum hydrocarbons (TPH).[2] Copolymers reduce the oil content up to 85%.[3] Zeolites are efficient in removing BTEX compounds.[4] A multi-stage adsorption and separation system was developed, for example, by EARTH Canada Corporation to recover dispersed oil droplets in water, whose size is greater than 2 microns.[5]

4.1.2. Sand filters They are generally used to remove metals from produced water. Process requires series of pre-treatment steps such as pH adjustment, an aeration unit and a solid separation unit. The removal efficiency is as high as 90%.[6]

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4.1.3. Cyclones A compact floatation unit (CFU) could remove dispersed oil from 50% to 70% using a centrifugal force. [7]The major drawback of using a cyclone is its low efficiency and inability to remove dissolved components.[8]

4.1.4. Evaporation Evaporation does not require chemical treatment which eliminates the risk of secondary sludge handling. It also does not require highly skilled labor. On the other hand, the requirement of energy is very high which increases the operating cost. The energy consumption could be brought down by reusing hot vapor to heat the fresh feed.[9]

4.1.5. Dissolved air precipitation (DAP) In this process, water at 500 kPa(for example) is saturated with air in a packed column separator. The pressure is released into the water column which causes the formation of air bubbles. It induces the flotation of aliphatic and aromatic hydrocarbons, and removes the aliphatic compounds more efficiently than aromatic compounds.[10]

4.1.6. C-TOUR It is a patented technology that uses liquid condensate to extract dissolved components from produced water. In field trials, the removal efficiency of dispersed oil was found to be 70%.[7]

4.1.7. Freeze-thaw/evaporation This technology uses the principle of solubility dependency on temperature. When the solution is cooled below the freezing point of the solvent but not below the depressed freezing point of the solution, relatively pure crystals of solvent and unfrozen concentrated solutions are obtained. If we couple this process with conventional evaporation, large volumes of clean solvent could be obtained. The process is capable of removing 90% of Total Recoverable Petroleum Hydrocarbons (TRPH). But it has several limitations like the requirements of sub-zero ambient temperatures and large land surface.[11]

4.2. Chemical treatment 4.2.1. Chemical precipitation The suspended solids and colloidal particles could be removed by coagulation and flocculation. Several coagulants like modified hot lime, FMA (a mixed metal polymer), Spillsorb, calcite and ferric ions were used as coagulant to treat produced water. The disadvantages of this process are its ineffectiveness for dissolved components and the increased concentration of metals in the sludge formed.[12, 13]

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4.2.2. Chemical oxidation It uses a combination of strong oxidants (e.g: O3 and H2O2), irradiation (e.g: UV) and a catalyst (e.g: photocatalyst), and oxidizes the organic components to their highest stable oxidation states.[14]

4.2.3. Electrochemical process Almost 90% of BOD and COD could be removed from produced water in a short time (of the order of 6 minutes) by using an active metal and graphite as an anode and iron as a cathode. During the process, Mn2+ is formed, which oxidizes and coagulates the organic contaminants.[15]

4.2.4. Photocatalytic treatment The pH of the solution is increased to a value of 11 by adding soda. The photochemical reaction was then carried out on the supernatant obtained from the flocculation unit. Titanium dioxide is usually used as photocatalyst. The COD removal efficiency and toxicity reduction were found to be higher in photoelectrocatalysis than that in photocatalysis.[16]

4.2.5. Fenton process Nearly 95% of COD and dispersed oil content can be reduced by combining flocculation with the Fenton oxidation adsorption process. The flocculent used is poly-ferric sulfate. [17]

4.2.6. Treatment with ozone Sonochemical oxidation could destroy BTEX compounds but the addition of hydrogen peroxide does not improve the efficiency. The process requires high initial and operating cost. [18]

4.2.7. Room temperature ionic liquids The hydrophobic room temperature ionic liquids remove certain soluble organic components efficiently, but not much of the other contaminants. Hence, the screening of ionic liquids depends on the constituents of produced water.[19]

4.2.8. Demulsifiers Some surfactants used as production chemicals are responsible for the stabilization of oilwater emulsions. They reduce the oil-water interfacial tension. Demulsifiers are surfaceactive agents that would disrupt the effects of surfactants. But a number of solids like silts, iron sulphide and paraffin, etc., present in the crude oil complicate the process.[20]

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4.3. Biological treatment The produced water could be treated with aerobic as well as anaerobic microorganisms. The microorganisms disintegrate the organic and ammonia compounds, but could not treat dissolved solids.[21] The COD removal efficiency increased up to 90% while treating produced water with Bacillus sp.[22]

5. Membrane treatment processes Conventional treatment methods are capable of removing suspended particles with particle size of 5.0 or above.The disposal and reinjection regulations are becoming more stringent and the conventional methods are not able to treat produced water which can meet these regulations.[23] The general specification for acceptable quality of oil-fields produced water for discharging into surface water (or re-injection) are less than 42 mg/L of oil/water, and less than 10 mg/L of Total Suspended Solids (TSS).[24] Conventional treatment processes are not able to meet these water effluent standards. New technologies should be utilized to separate both fine particles and dissolved components.[23] Membrane processes are a rather new separation process for treatment of produced water. Membrane separation processes, including microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO), are able to treat produced water and generate water with high standards to meet regulations. The driving force of the above mentioned membranes processes is pressure gradient.[23]

5.1. Advantages of membrane technology Membrane technologies have some advantages that make them popular for produced water treatment processes:[25, 26]            

sludge reduction high quality of permeate smaller space needed ease of operation minimal impact on permeate quality with variation in feed water quality little or no chemicals required possibility for recycling of waste streams possibility for having an automated plant moderate capital costs ability to be combined easily with other separation processes low energy consumption continuous separation

But there may be some drawbacks for using membrane processes including concentration, polarization/membrane fouling, low selectivity or low flux and low membrane lifetime.

State of the Art Treatment of Produced Water 205

According to the above mentioned advantages, that membrane separation processes can, in some circumstances, be viable for treatment of produced water.[25]

5.2. Membrane properties There are different types of membrane processes, membrane materials, and feed water compositions, but the main goal of preparing the membranes is the same. An ideal membrane should:   

be mechanically resistant have a high permeate flowrate have a high selectivity for a specific component

Having high permeate flowrate means having large pore sizes. A high level of selectivity for a certain component is achievable with small pore sizes and the range of pore sizes should be narrow. The last two parameters present a dilemma, as one is in conflict with the other. The membrane mechanical resistance depends on the membrane thickness. Therefore the membrane should have a thin layer of material (the selective layer), narrow pore sizes, and high porosity. [27] According the type of materials and mechanism of separations, membranes may be categorized as porous or dense. Separation of dense membranes is based on physicochemical interaction of permeate and the membrane material. Separation mechanism of porous membranes is based on the mechanical separation by size of permeates and pore sizes of membrane (sieving).[27]

5.3. Types of membranes Membranes can be generally classified based on their structure or morphology. The detailed classification of membranes is reported in Table2. Symmetric and asymmetric membranes are two classes. Symmetric membranes have different types including isotropic microporous, nonporous dense membrane, and electrically charged membranes. Asymmetric membranes are divided into Loeb-Sourirajan anisotropic, thin-film composite anisotropic, and supported liquid membranes.[28] Membranes can also be classified based on the type of materials like ceramic, inorganic, and composite membranes.

5.3.1. Polymeric membranes Polymeric membranes have some advantages including high efficiency for the removal of particles, emulsified and dispersed oil; small size; low energy requirements, and being cheaper than ceramic membranes. They also have some disadvantages including the inability to separate volatile and low molecular weight compounds, fouling problems due to oil, sulfide or bacteria which may be required to be cleaned daily, an inability to be used at temperatures above 50 ℃, and they also create the possibility of having radioactive byproduct in the effluent and need for some pre-treatment processes. [26]

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Process

Mechanism of separation

Material/Type

Typical Objective

Microfiltration Separation by sieving Polymeric and (MF) through macropores inorganic / (>50 nm) Porous

Removal of suspended solids, large organic molecules, and large colloidal particles including microorganisms (used for reducing colloidal suspensions and turbidity)

Ultrafiltration Separation by sieving Polymeric and (UF) through mesopores inorganic / (2-50 nm) Porous

Removal of large dissolved solute molecules and suspended colloidal particles, including bacteria and macromolecules such as proteins

Nanofiltration Separation through Polymeric / (NF) combination of charge Dense rejection, solubilitydiffusion and sieving through micropores (